This invention relates to water-soluble nanocrystalline materials that emit energy over a narrow range of wavelengths. In particular, the invention relates to water-soluble nanocrystals that emit light in the visible energy range.
Semiconductor nanocrystals (quantum dots) whose radii are smaller than the bulk exciton Bohr radius constitute a class of materials intermediate between molecular and bulk forms of matter. Quantum confinement of both the electron and hole in all three dimensions leads to an increase in the effective band gap of the material with decreasing crystallite size. Consequently, both the optical absorption and emission of quantum dots shift to the blue (higher energies) as the size of the dots gets smaller.
Bawendi and co-workers have described a method of preparing monodisperse semiconductor nanocrystals by pyrolysis of organometallic reagents injected into a hot coordinating solvent (J Am. Chem. Soc., 115:8706 (1993)). This permits temporally discrete nucleation and results in the controlled growth of macroscopic quantities of nanocrystals. Size selective precipitation of the crystallites from the growth solution can provides crystallites with even narrower size distributions. The narrow size distribution of the quantum dots allows the possibility of light emission in narrow spectral widths.
In an effort to improve the photoluminescent yield of the quantum dots, the nanocrystal surface has been passivated by reaction of the surface atoms of the quantum dots with organic passivating ligands, so as to eliminate energy levels at the surface of the crystallite which lie within the energetically forbidden gap of the bulk interior. These surface energy states act as traps for electrons and holes which degrade the luminescence properties of the material. Such passivation produces an atomically abrupt increase in the chemical potential at the interface of the semiconductor and passivating layer (See, A. P. Alivisatos, J Phys. Chem. 100:13226 (1996)). Bawendi et al. (J. Am. Chem. Soc. 115:8706 (1993)) describe CdSe nanocrystals capped with organic moieties such as tri-n-octyl phosphine (TOP) and tri-n-octyl phosphine oxide (TOPO) with quantum yields as high as 20% in organic solvents such as toluene. See also, thesis of Christopher Murray, xe2x80x9cSynthesis and Characterization of II-VI Quantum Dots and Their Assembly into 3-D Quantum Dot Superlatticesxe2x80x9d, Massachusetts Institute of Technology, September, 1995; and Kuno et al. (J. Phys. Chem. 106(23):9869 (June, 1997)).
Although semiconductor nanocrystals prepared as described by Bawendi and co-workers exhibit near monodispersity, and hence, high color selectivity, the luminescence properties of the material is process dependent. The stability of the photoluminescent property of the nanocrystal is a function of the nature of the passivating species coating the outer surface of the nanocrystal. Known organically coated nanocrystals are not robust and exhibit degradation of photoluminescent yield in solution. This is likely due to dissociation of the passivating layer from the surface of the quantum dot or degradation of the passivating layer resulting in degradation of the semiconductor surface.
Passivation of quantum dots using inorganic materials also has been reported. Particles passivated with an inorganic coating are more robust than organically passivated dots and have greater tolerance to processing conditions necessary for their incorporation into devices. Previously reported inorganically passivated quantum dot structures include CdS-capped CdSe and CdSe-capped CdS (Than et al., J Phys. Chem. 100:8927 (1996)); ZnS grown on CdS (Youn et al., J Phys. Chem. 92:6320 (1988)); ZnS on CdSe and the inverse structure (Kortan et al., J Am. Chem. Soc. 112:1327 (1990)); ZnS-capped CdSe nanocrystals (M. A. Hines and P. Guyot-Sionnest, J Phys. Chem. 100:468 (1996); ZnSe-capped CdSe nanocrystals (Danek et al., Chem. Materials 8:173 (1996) and SiO2 on Si (Wilson et al., Science 262:1242 (1993)).
Kortan et al. describes a ZnS capped-CdSe quantum dot which has a layer of thiolphenyl groups bound to the outer surface. The thiolphenyl groups were used to passivate the surface and to allow the clusters to be isolated in powder form. Lawless et al. reported the preparation of CdS semiconductor nanocrystals capped with bifunctional mercaptocarboxylic acids HS(CH2)nCOOH, where n=1-3. TiO2 particles were attached to the CdS dots through the functional carboxylic acid group of the bifunctional capping moiety in order to promote interparticle electron transfer between dissimilar semiconductor particles.
The quantum dots described above are soluble or dispersible only in organic solvents, such as hexane or pyridine. Many applications which rely on the fluorescent emission of the quantum dots require that the quantum dots be water-soluble.
Many reported water-soluble quantum dots suffer from significant disadvantages which limit their wide applicability. For example, Spanhel et al. discloses a Cd(OH)2-capped CdS sol (J. Am. Chem. Soc. 109:5649 (1987)); however, the photoluminescent properties of the sol was pH dependent. The sol could be prepared only in a very narrow pH range (pH8-10) and exhibited a narrow fluorescence band only at a pH of greater than 10. Such pH dependency greatly limits the usefulness of the material; in particular, it is not appropriate for use in biological systems.
Other groups have replaced the organic passivating layer of the quantum dot with water-soluble moieties; however, the resultant derivatized quantum dots are not highly luminescent. Short chain thiols such as 2-mercaptoethanol and 1-thio-glycerol have been used as stabilizers in the preparation of water-soluble CdTe nanocrystals. See, Rogach et al., Ber. Bunsenges. Phys. Chem. 100:1772 (November, 1996) and Rajh et al., J Phys. Chem. 97:11999 (November 1993). Other more exotic capping compounds have been reported with similar results. See, Coffer et al. (Nanotechnology 3:69 (April, 1992) which describes the use of deoxyribonucleic acid (DNA) as a capping compound. In all of these systems, the coated quantum dots were not stable and photoluminescent properties degraded with time.
The unavailability of aqueous suspensions or solutions of quantum dots with sharp photoluminescent emissions limits their application in a variety of water-based applications, such as biological applications. In addition, aqueous solutions can often be very aggressive chemical systems and many of the known water-soluble quantum dots systems degrade, mainly by photoanodic decomposition at the semiconductor surface interface, during long exposure times in water. Thus there remains a need for water-soluble semiconductor nanocrystals which may be prepared as stable, robust suspensions or solutions in aqueous media. There is also a need for water-soluble quantum dots capable of energy emission with high quantum efficiencies, which possess a narrow particle size (and hence with narrow photoluminescence spectral range).
It is the object of the invention to provide water-soluble semiconductor nanocrystals (quantum dots) which overcome the limitations of the prior art and which exhibit high quantum yields with photoluminescence emissions of high spectral purity.
It is yet a further object of the present invention to provide a quantum dot which is readily soluble in aqueous systems and demonstrates chemical and electronic stability therein.
It is yet a further object of the invention to provide a water-soluble quantum dot derivatized to provide linking or coupling capability.
In one aspect of the invention, a water-soluble semiconductor nanocrystal capable of energy emission is provided. The nanocrystal includes a quantum dot having a selected band gap energy overcoated with a layer of a material having a band gap energy greater than that of the quantum dot and with appropriate band offsets. An outer layer is found at the outer surface of the overcoating layer. The outer layer includes a compound having at least one linking group for attachment of the compound to the overcoating layer and at least one hydrophilic group spaced apart from the linking group by a hydrophobic region sufficient to prevent electron charge transfer across the hydrophobic region.
The outer layer of the nanocrystal may comprise an organic compound. The organic compound may be comprised of moieties selected to provide solubility in an aqueous medium, such as a long chain hydrocarbon terminating in a moiety having affinity for an aqueous medium, and a moiety that demonstrates an affinity to the quantum dot surface. The affinity for the nanocrystal surface promotes coordination of the organic compound to the quantum dot outer surface and the moiety with affinity for the aqueous medium stabilizes the quantum dot suspension.
In preferred embodiments, the compound may have the formula, HzX((CH2)nCO2H)yand salts thereof, where X is S, N, P or Oxe2x95x90P; nxe2x89xa76; and z and y are selected to satisfy the valence requirements of X. In other preferred embodiments, the compound may have the formula, 
where X and Xxe2x80x2 are the same or different and are selected from the group of S, N, P or Oxe2x95x90P; Y is a hydrophilic moiety; and Z is a hydrophobic region having a backbone of at least six atoms. X and Xxe2x80x2 may include other substituents in order to satisfy the valence requirements, such as for example, amines, thiols phosphine and phosphine oxides, substituted by hydrogen or other organic moieties. In addition, the atoms bridging X and Xxe2x80x2 are selected to form a 5-membered to 8-membered ring upon coordination to the semiconductor surface. The bridging atoms are typically carbon, but may be other elements, such as oxygen, nitrogen, and sulfur. Y may be any charged or polar group, such as carboxylates, sulfonates, phosophates, polyethylene glycol and ammonium salt, and the like. Z is a hydrophobic group, typically an alkyl group or alkenyl group, but may also include other atoms, such as oxygen, sulfur, silicon or nitrogen. Z may be further modified as described herein to provide attractive interactions with neighboring ligands.
In yet another preferred embodiment, the compound may have the formula, 
where X, Xxe2x80x2 and Xxe2x80x3 are the same or different and are selected from the group of S, N, P or Oxe2x95x90P; Y is a hydrophilic moiety; and Z is a hydrophobic region having a backbone of at least six atoms. X, Xxe2x80x2 and Xxe2x80x3 may include other substituents in order to satisfy the valence requirements, such as for example, amines, thiols, phosphine and phosphine oxides, substituted by hydrogen or other organic moieties. In addition, the atoms bridging X, Xxe2x80x2 and Xxe2x80x3 are selected to form a 5-membered to 8-membered ring upon coordination to the semiconductor surface. The bridging atoms are typically carbon, but may be other elements, such as oxygen, nitrogen, and sulfur. Y may be any charged or polar group, such as carboxylates, sulfonates, phosphates, polyethylene glycol and ammonium salt, and the like. Z is the hydrophobic group, typically and alkyl group or alkenyl group, but may also include other atoms, such as oxygen, sulfur, silicon or nitrogen. Z may be further modified as described herein to provide attractive interactions with neighboring ligands.
In yet another embodiment of the invention, the compound may be a block copolymer. A first block is provided that includes a pendant group capable of functioning as a linking moiety, Y. A second block is provided that includes a pendant group capable of functioning as a hydrophilic group, X. The polymer block serves as a hydrophilic region. In a preferred embodiment, the compound may have the formula, 
where X are the same or different and elements selected from the group of S, N, P or Oxe2x95x90P; and Y is a hydrophilic moiety, such as carboxylates, sulfonates, phosphates, polyethylene glycol and ammonium salt, and the like. X may include other substituents in order to satisfy the valence requirements, such as for example, amines, thiols, phosphine and phosphine oxides, substituted by hydrogen or other organic moieties. The terminal groups R and Rxe2x80x2 may be any moiety. In particular, it is desirable for R to be a polar moiety due to its proximity to the hydrophilic block. Similarly, it is desirable for Rxe2x80x2 to be non-polar moiety due to its proximity to the hydrophobic block. m and n are selected in relation to the number of available coordinating sites on the surface of the quantum dot. It is desirable that m be no greater than the number of available coordinating sites and preferably no greater than one-fourth of available coordinating sites. In typical applications, m is in the range of about 3 to 100. The value of n is typically chosen to be commensurate with the value for m. Thus, it is desirable that n be no greater than the number of available coordinating sites and preferably no greater than one-fourth of available coordinating sites. In typical applications, n is in the range of about 3 to 100.
In another aspect of the invention, a water-soluble semiconductor nanocrystal is provided in which the water solubilizing layer is a bilayer, having a first layer of the bilayer having affinity for the overcoating layer and a second layer of the bilayer having a hydrophobic region adjacent to the first layer and terminating in a hydrophilic group. The bilayer may include a coordinating lyophilic compound used in the manufacture of the semiconductor nanocrystal as the first layer and a surfactant as the second layer.
By xe2x80x9cquantum dotxe2x80x9d as that term is used herein, it is meant a semiconductor nanocrystal with size-dependent optical and electrical properties. In particular, the band gap energy of a quantum dot varies with the diameter of the crystal. In the naming convention used herein to refer to coated nanocrystals, the compound found within parentheses represents the core compound (i.e. the quantum dot), while the compound which follows represents the overcoated layer. xe2x80x9cQuantum yieldxe2x80x9d as that term is used herein, means the ratio of photons emitted to that absorbed, e.g., the photoluminescence quantum yield.
In other embodiments of the invention, the coated nanocrystal is characterized in that the nanocrystal exhibits less than a 10% and preferably less than 5%, rms deviation in diameter of the core. The nanocrystal in an aqueous environment preferably exhibits photoluminescence having quantum yields of greater than 10%, and most preferably in the range of about 10 to 30%.
Upon exposure to a primary light source, the quantum dot emits energy of a wavelength characteristic of its composition and size. The water-soluble layer of the invention may be used with nanocrystals having various combinations of nanocrystal core and overcoating. The invention permits the preparation of a variety of water-soluble nanocrystals having a very narrow particle size distribution and exhibiting improvements in color purity and intensity of their photoluminescent emissions, as well as demonstrating robustness and stability in water-based suspensions and solutions.
These and other features and advantages of the invention are set forth in the description of the invention, which follows.